The present invention relates to dual-circuit brake control arrangements in general, and more particularly to control arrangements of this type which incorporate a master cylinder device supplying two separate braking circuits with pressurized hydraulic braking fluid upon actuation of a brake pedal.
There are already known dual-circuit control arrangements of this type inc1uding a master cylinder device which defines a first pressure chamber slidably accommodating a first master piston, and a second chamber slidably accommodating a second master piston. In many conventional constructions of the control arrangements of this type, the second master piston is movable in the same direction as the first piston so long as the two separate braking circuits are intact, in that the pressure which develops in a first pressure compartment delimited by the first master piston in response to movement of the first piston in its actuating direction acts on that side of the second master piston which faces away from its associated second pressure compartment to displace the second master piston in the same actuating direction and thus to pressurize the hydraulic braking fluid present in the second pressure compartment. It is also known to mechanically actuate the second master piston in the event that the brake circuit supplied with the hydraulic braking fluid from the first pressure compartment becomes defective.
Tandem master cylinder devices as known, for instance, from the ATE Brake Handbook 6th edition, 1979, page 47, are being currently used as the basic equipment for dual-circuit hydraulic braking systems. In such tandem master cylinder devices, two master cylinders are united in a common housing, being arranged in series or in tandem with one another. Each of the master cylinder parts of this common housing slidably accommodates a master piston, so that the master pistons are arranged in tandem with one another as well. A first of these master pistons has a piston rod or extension which is acted upon by a brake actuating force upon brake pedal depression, so that the first master piston is displaced in its actuating direction and pressurizes the hydraulic fluid which is present in the first pressure compartment situated in front of the first master piston. When the first piston is displaced in the actuating direction, that is, toward the second master piston, the first piston first moves beyond a feeding port which initially communicated the first pressure compartment with a brake fluid supply reservoir, so that communication of the first pressure compartment with the supply reservoir is interrupted and the hydraulid brake fluid present in the first pressure compartment is pressurized during the further movement of the first master piston in the actuating direction. Since the thus raised pressure of the hydraulic braking fluid in the first pressure compartment is uniformly effective in all directions, it will also act on the second master piston to displace the same in the actuating direction and eventually to pressurize the hydraulic braking fluid present in the second pressure compartment situated ahead of the second master piston as well. This additional pressurization takes place after the second piston has been moved beyond its associated feeding port which serves the same purpose as the feeding port associated with the first piston. Thus, the pressure in both the first and second pressure compartments will be about the same.
The pressurized hydraulic fluid is admitted, through respective outlet ports, into the respectively associated separate brake circuits. Now, should a leak develop in the hydraulic braking circuit associated with the first pressure compartment, the pressure in the latter will not rise in response to the movement of the first master piston in the actuating direction, inasmuch as the hydraulic braking fluid will escape through the leak. When this happens, an engaging portion of the first master piston will engage a corresponding engaging portion of the second master piston, so that the actuating force is mechanically transmitted to the second master piston after the first master piston has moved through a predetermined distance, whereafter the pressure of the fluid present in the second pressure compartment and thus in the still intact braking circuit is increased. This expedient will keep the braking circuit associated with the second pressure compartment fully effective. On the other hand, if the leak occurs in the braking circuit associated with the second pressure compartment, no pressure build-up will take place in the second pressure compartment. Thus, no resistance will be offered to the movement of the second master piston in the actuating direction upon movement of the first master piston in the actuating direction until an engaging portion of the second master piston abuts a stop which is stationary relative to the master cylinder housing. Thereafter, the first master piston will pressurize the hydraulic braking fluid in the first pressure compartment and, consequently, the braking circuit associated with the first pressure comparment will remain fully effective.
Tandem master cylinder device have generally proven their utility even in conjunction with brake boosters, inasmuch as the actuating force with which either of the brake pedal or the brake booster acts on the first master piston is hydraulically transmitted to the second piston. This means that a hydraulic equalizing action takes place between two independent braking circuits due to the action of the second master piston, so long as both braking circuits are intact. Under these circumstances, the pressure conditions in the two separate braking circuits are almost the same as if the second master piston were not provided at all. In other words, except for a minute difference attibutable to the existence of frictional forces, the braking pressures in the two separate braking circuits are virtually the same.
As advantageous as the conventional tandem master cylinder devices are in many respects, they still have the disadvantage of being relatively long and thus requiring a correspondingly dimensioned available space for mounting.
There is already known, from the German Pat. No. 17 80 218, a dual-circuit master cylinder device including two master pistons which are received for axial sliding in two separate cylindrical bores extending parallel to one another. In this arrangement, which is often referred-to as twin arrangement, an actuating member acts on the two master pistons through a balancing bore equalizing beam. When the separate braking circuits are intact the master cylinder device of this construction acts in the same manner as a tandem master cylinder device, provided that the effective surfaces of the master pistons are half as large as those of the tandem-type master pistons. However, the functioning of the twin arrangement of this type becomes problematical when one of the braking circuits fails, inasmuch as the entire pedal force will act only on one half of the previously effective master piston surface and thus a different transmission ratio between the brake force on the brake pedal and the pressure in the intact braking circuit will be obtained. In other words, the same brake force applied by the driver of the vehicle in which the brake system is being used will result in a two-fold increase in the pressure in the still intact braking circuit after the other braking circuit has failed as compared to the situation existing prior to the failure and, consequently, the braking action of the brakes incorporated in this still intact braking circuit will be excessive.
Moreover, one of the main disadvantages of the aformentioned twin arrangement is the relatively costly and complicated construction of the lever mechanism that is being used to compensate for different movements of the master pistons. It is necessary to so construct this compensating mechanism as to achieve blocking of the same upon failure of one of the braking circuits to render it possible to still effectively operate the intact braking circuit. So far, no practical constructions capable of accomplishing this purpose have been developed.
There has also already been proposed, in the German published patent application No. DE-OS 26 15 805, a brake unit having two brake cylinders which are arranged side by side. However, in this arrangement, one of the brake cylinders accommodates an auxiliary piston which serves as a servomechanism in that it is acted upon by the brake pedal force to pressurize a hydraulic fluid which is then supplied to that side of the actual master piston which faces away from the pressure compartment. Upon failure of the servo-mechanism, the single master piston is directly acted upon by the brake pedal force. This arrangement is capable of controlling the pressure of the hydraulic braking medium only in one braking circuit, while another braking circuit is directly connected to a brake booster chamber.
Furthermore, the published German application No. DE-OS 27 45 514 discloses a master cylinder assembly for vehicle hydraulic braking systems. In this construction, two master cylinders are arranged in parallel side by side, in a common housing. In contrast to the present invention, however, each of these master cylinders has to be individually operated by its own brake pedal for braking the vehicle when both brake pedals are simultaneously depressed, and for steering the vehicle by independently selectively depressing the respective brake pedals. Thus, this arrangement, like those discussed above, leaves much to be desired as to its practicality, simplicity and utility.